1. Field of the Invention
The invention relates to the field of electrical/electronic arts. More particularly the invention lies in the area of microwave power divider, printed circuit technology. More specifically, the invention discloses a method to efficiently combine two conductor strips of a power divider/combiner on opposite sides of a dielectric substrate in a three substrate layer, i.e. in a stripline device, into a single conductor strip on one side of the dielectric substrate.
2. Description of the Prior Art
Power dividers and combiners have a countless number of applications in various microwave systems and comprise narrow band, wide band, stepped impedance, tapered line, coaxial line, and stripline three port power dividers. Unfortunately, none of these prior art designs have been realizable in stripline with the following characteristics:
1. Reproducibility at low cost; PA1 2. Power balance or tracking at the outputs better than 0.1 dB over frequency range; PA1 3. Negligible insertion loss over what is due to that length of stripline; PA1 4. Directivity of device decibel minimum, i.e. directivity being defined as the difference between the output port isolation and the insertion loss of the device; and PA1 5. Minimum length of the power divider for a given degree of match at each of the three terminals.
Typical power divider isolation requirements basically rule out from consideration any heretofore known design employing stepped impedance lines or discrete loading between the two coupled lines. Also ruled out is any design that does not consider the two lines of the device to be coupled lines; i.e. coupling exists due to the required close proximity of the two lines leading to the power divider.
The minimum length, i.e. the minimum total insertion loss, requirement of a power divider of the type disclosed herein for a given degree of match dictates that the transformer be a Chebyshev type, not simply a linear or other type of taper.
Coplanar, stripline, Chebyshev-tapered, power dividers have been designed that perform quite well. In one such design, the slot between the coupled lines, which contains a lossy film attenuator, varied non-linearly with the link because a constant odd mode impedance of 50 ohms was used; however, to cut and shape the lossy material and insert it into the slot without getting significant attenuation of the even mode was extremely difficult to accomplish from a practical standpoint. The slightest overlap of the lossy film with the stripline induced several decibels of loss for the even mode. Such a power loss makes accurate tracking or power balance virtually impossible to achieve due to the critical alignment of the lossy film material with the stripline conductors.
Another power divider was designed similar to the above-described technique except that the odd mode impedance was chosen so that the slot between the coupled lines became a triangular wedge, facilitating accurate cutting of the lossy film material. This allowed considerably simpler alignment of the lossy film with the stripline conductors without excessive overlap. Unfortunately, it was still very difficult to insert the lossy film in such a way that the insertion loss was low while yielding good balance and high isolation.
Based on the assumption that a film between two coplanar strips was too critical for placement, a subsequent design involved a broadside coupled stripline power divider. The constraint on the mode for a Chebyshev-tapered device was that the unloaded impedance be 50 ohms as in conventional quadrature couplers. Therefore, the odd mode impedance was reduced below the theoretical value in the loaded region, thus slightly degrading the match as seen from the output terminals. Holes were drilled and resistively loaded with dielectric spacers separating the offset parallel coupled striplines while keeping the loading material reasonably well-confined between the coupled conductors. Again, however, it was very, very difficult to attain good power division and isolation with low insertion loss in this type device in part because the conductor edges of the coupled line were severed by the drill when loading the dielectric spacer and later could not be sufficiently well repaired. However, power handling capability of the broadside coupled line configuration proved superior to the earlier coplanar etched coupled configuration.
A subsequent design, disclosed in U.S. Pat. No. 3,886,498 by Mosko and Corzine, provided a broadside power divider with loading only in the completely overlapping region of the conductor strips. As before, the even mode impedance of the coupled stripline was selected to taper from 100 to 50 ohms for a 50 ohm input impedance at all three terminals as a Chebyshev transformer. Over a suitable part of the total length of the device the odd mode impedance was required to be sufficiently low so that the coupled lines were exactly overlapped, where the s/b ratio was previously selected for compatibility with other stripline components etched on the same stripline sandwich and interconnected with the power divider; "b" conventionally being the total thickness of a three layer dielectric sandwich and "s" being the central dielectric thickness. This allowed loading between the coupled lines, through the s layer (s-spacer) with suitable load material in order to absorb/attenuate the odd mode and refilling the conductor strip holes with similar conductive material. This could be easily accomplished since any drilling, milling, etc., could be done without severing the outside edges of the coupled lines. In addition, the even mode electric field lines are nonexistent between the coupled strips, thereby guaranteeing least loss for the even mode. Thus, a highly symmetrical and effective lossy transmission line for the odd mode, and simultaneously, lossless even mode line could be realized.
Although the foregoing prior art devices were indeed substantial advances in state-of-the-art at the time and were furthermore competent for their design application, there remains a continuing need to efficiently integrate certain stripline oriented microwave networks. Millican networks conventionally have a broadside, i.e. parallel/coplanar conductor strips on opposite sides of a dielectric, power divider requirement, and there exists a need to integrate said power divider with the rest of the microwave circuit; i.e. the two broadside conductor strips must be electromagnetically, efficiently joined into a single conductor strip on the substrate face on which the rest of the microwave circuit lies. Prior art broadside stripline devices encountered substantial difficulty in accomplishing such a transition. The invention described in the following disclosure overcomes this unilateral limitation of prior art devices.